you need the tamiya one since you are only changing the spur and slipper pads. You are still using the main shaft, spring, tophat washer and nut. You can fit the b4 81t spur on BUT you need to dremel out the gear cover a little (because the 81t B4 spur is larger than the 82t tamiya spur) but the 78t b4 should fit in there fine.
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